A small amount of radioactive thorium (Th-231 and Th-234) is produced from the radiolytic decay of uranium. The thorium is normally in equilibrium with the uranium and it stays with the uranium throughout normal uranium processing operations. Because of its low concentration and the shielding provided by the uranium, the radioactivity from the thorium associated with the uranium is normally not a problem. However, when uranium is purified by solvent extraction the uranium is extracted into the organic phase and the thorium remains in the aqueous effluent (raffinate) solution. Although the amount of thorium associated with the uranium is very small, it has a high specific activity and therefore significantly increases the radioactivity of the liquid effluent.
Thorium is normally removed from the solvent extraction liquid effluent by chemical separation followed by aging of the separated thorium-containing solids. Alternately, the entire liquid effluent can be held until the thorium decays to an acceptable level for residual radiation. Because Th-234 has a half-life of 24 days, very large tanks or surface impoundments are normally needed to hold the entire liquid effluent stream for the required decay time. Since Th-231 has a half-life of only 25 hours, it is not usually a factor in determining the required decay time of the thorium-containing solids or liquid effluent.
Elimination of the need to store and age the thorium-containing solids or liquids would greatly simplify the design of thorium treatment systems and eliminate the need for potentially dangerous handling and treatment of the thorium.